1. Field of the Invention
This invention relates to the field of warm-up air delivery control for an internal combustion engine, and in particular to air delivery control during the engine start and warm-up periods generally referred to as the fast idle control, which adjusts the idle air flow to the engine controlling the engine's idle speed during the transitional warm-up period.
2. Prior Art
The requirement for a cold engine to have a substantially faster idle speed than a warm engine in order to overcome increased viscous and frictional loads encountered in a cold engine is recognized. This problem was met early in the development of internal combustion engines by what is now conventionally referred to as fast-idle controls. These controls are primarily open-looped controls having an operative duration based on the temperature of the engine or a fixed time period. Early fast-idle controls employed thermally expansive or temperature responsive devices such as bi-metallic springs to set the position of a fast idle cam controlling the idle position of the throttle in the primary air delivery system. U.S. Pat. No. 2,420,917 "Carburetor" by R. W. Sutton et al represents a typical device of the type described above. Fast idle controls of the types taught by Sutton above and variations thereof have found wide acceptance in the automotive and allied fields and are still being used today. An alternate to controlling the position of the throttle to achieve fast idle during engine warm-up, a variety of systems can be found in the prior art having a valve controlled throttle bypass air passage which admits auxiliary or idle air into the manifold at a point downstream of the closed throttle. The Eckert et al U.S. Pat. No. 3,645,509 suggests a system using an electrically heated poppet or slide valve to control the quantity of idle air being admitted into the manifold as a function of time based on the initial temperature of the engine independent of the actual rate at which the engine warms up. In another system suggested by Charron U.S. Pat. No. 3,739,760 the idle air flow is thermostatically controlled as a function of engine temperature. The Charron system also provides means for premixing a proportional quantity of fuel with the idle air prior to entering the intake manifold.
Closed loop systems for controlling an engine to run at a predetermined or operator set speed are well known in the art and are commercially available for a wide variety of automotive and aircraft applications. Although the majority of these engine speed control systems are designed to control the engine at speeds much higher than curb idle speed, Croft in U.S. Pat. No. 3,661,131 suggests that such a speed control system can be used to control the idle speed of the engine. Croft, however, only teaches the use of a fixed reference for controlling the idle speed of the engine and is ineffective as a control during the transient warm-up period where the idle speed required to sustain the operation of the engine is continuously changing.
The idle operating speed of any given internal combustion engine is primarily a function of three parameters -- air, fuel and load. In the prior art systems having fast idle controls the load on the engine during the warm-up period is only considered as a function of the engine's temperature independent of the subsequent mechanical load to which the engine will be subjected during the warm-up period. A typical example of a variable load is found in automotive applications where prior to the engine warming up to its normal operating temperature, the operator may engage the engine with the transmission and ultimately the drive wheels while the engine is still cold and in its fast idle mode of operation. In order to prevent the engine from stalling, the fast idle control as taught by the prior art must be adjusted to accommodate the highest engine load anticipated which is significantly higher than that required to sustain the operation of the engine without the additional load. As a result, these open-loop systems are inefficient and wasteful adding to the already excessive exhaust pollution. On the other hand, the speed control systems of the prior art only considered the load and not the warm-up requirements of the engine.
The invention is directed to a closed-loop fast idle control which continuously controls the idle air delivery to the engine during the warm-up period to maintain the idle speed of the engine at a predetermined speed as a function of the engine temperature. Being a closed loop system, the disclosed auxiliary air delivery system automatically compensates for changes in the engine load whether it be internal to the engine itself or an external load, and changes in the idle speed required to sustain the operation of the engine as a function of its operating temperature.